1. Field of the Invention
The present invention is related to a liquid crystal display device and a related driving method, and more particularly, to a liquid crystal display device with adaptive charging/discharging time and a related driving method.
2. Description of the Prior Art
Liquid crystal display (LCD) devices, characterized in low radiation, small size and low power consumption, have gradually replaced traditional cathode ray tube (CPT) displays and been widely used in electronic products such as notebook computers, personal digital assistants (PDAs), flat panel TVs, or mobile phones. An LCD device displays images by driving the pixels of the panel using a source driver and a gate driver. Based on driving modes, the LCD device can adopt single-gate pixel layout or double-gate pixel layout. When compared to an LCD panel having single-gate pixel layout under the same resolution, the number of gate lines is doubled and the number of data lines is halved in an LCD panel having double-gate pixel layout, therefore requiring more gate driver chips and fewer source driver chips. Since gate driver chips are less expensive and consume less power, double-gate pixel layout can lower manufacturing costs and power consumption.
FIG. 1 is a diagram illustrating a prior art LCD device 100. The LCD device 100 includes an LCD panel 110, a source driver 120, a gate driver 130, and a timing controller 140. A plurality of data lines DL1-DLm, a plurality of gate lines GL1-GLn, and a pixel array are disposed on the LCD panel 110. The pixel array includes a plurality of pixel units P11-Pmn (m and n are positive integers) each having a thin film transistor switch TFT, a liquid crystal capacitor CLC and a storage capacitor CST. Each pixel unit is coupled to a corresponding data line, a corresponding gate line, and a common voltage VCOM. In the LCD device 100, the pixel units P11-Pmn receive data signals from corresponding data lines disposed at the left side. The timing controller 140 is configured to generate control signals for operating the source driver 120 and the gate driver 130, such as a start pulse signal VST, a horizontal synchronization signal HSYNC, and a vertical synchronization signal VSYNC. According to the start pulse signal VST and the vertical synchronization signal VSYNC, the gate driver 130 respectively outputs gate driving signals SG1-SGn to the gate lines GL1-GLn, thereby turning on the thin film transistor switches TFT in the corresponding rows of pixel units. According to the horizontal synchronization signal HSYNC, the source driver 120 respectively outputs data driving signals SD1-SDm related to display images to the data lines DL1-DLm, thereby charging the liquid crystal capacitors CLC and the storage capacitors CST in the corresponding columns of pixel units. In the LCD device 100, the type and polarity of each pixel unit are represented by “R” (red pixel), “G” (green pixel), B″ (blue pixel), “+” (positive polarity) and “−” (negative polarity) in FIG. 1. In order to achieve dot inversion in the LCD device 100, the data driving signals outputted to each pixel unit need to be inverted periodically, thereby consuming a lot of power.
Reference is made to FIG. 2 for a timing diagram illustrating the operation of the LCD device 100. In FIG. 2, SG represents the waveform of the gate driving signal, SD represents the waveform of the data driving signal, and VPIXEL represents the voltage level of the pixel unit. The grayscale value of a display image of the pixel unit is determined by the voltage difference between the data driving signal SD and the common voltage VCOM. During the charging period TC, the high-level gate driving signal turns on the thin film transistor switches TFT in the corresponding pixel units. The data signal SD can thus be written into the liquid crystal capacitor CLC and the storage capacitor CST in the corresponding pixel units, thereby changing the voltage levels of the corresponding pixel units. In high-resolution applications, the LCD device 100 needs to adopt more gate lines. Therefore, the charging period TC of each pixel unit is shortened and the pixel units may not have sufficient time to reach the predetermine level VGH or VGL.
FIG. 3 is a diagram illustrating another prior art LCD device 200. The LCD device 200 includes an LCD panel 210, a source driver 220, a gate driver 230, and a timing controller 240. A plurality of data lines DL1-DLm+1, a plurality of gate lines GL1-GLn, and a pixel array are disposed on the LCD panel 210. The pixel array includes a plurality of pixel units P11-Pmn (m and n are positive integers) each having a thin film transistor switch TFT, a liquid crystal capacitor CLC and a storage capacitor CST. Each pixel unit is coupled to a corresponding data line, a corresponding gate line, and a common voltage VCOM. The LCD device 200 adopts a zigzag layout in which the odd-numbered rows of pixel units P11-Pm1, P13-Pm3, . . . , P1(n−1)-Pm(n−1) receive data signals from corresponding data lines disposed at the left side, while the even-numbered rows of pixel units P12-Pm2, P14-Pm4, . . . , P1n-Pmn receive data signals from corresponding data lines disposed at the right side (assuming n is an even number). The timing controller 240 is configured to generate control signals for operating the source driver 220 and the gate driver 230, such as a start pulse signal VST, a horizontal synchronization signal HSYNC, and a vertical synchronization signal VSYNC. According to the start pulse signal VST and the vertical synchronization signal VSYNC, the gate driver 230 respectively outputs gate driving signals SG1-SGn to the gate lines GL1-GLn, thereby turning on the thin film transistor switches TFT in the corresponding rows of pixel units. According to the horizontal synchronization signal HSYNC, the source driver 220 respectively outputs data driving signals SD1-SDm+1, related to display images to the data lines DL1-DLm+1, thereby charging the liquid crystal capacitors CLC and the storage capacitors CST in the corresponding columns of pixel units. In the LCD device 200, the type and polarity of each pixel unit are represented by “R” (red pixel), “G” (green pixel), B″ (blue pixel), “+” (positive polarity) and “−” (negative polarity) in FIG. 3. In order to achieve dot inversion in the LCD device 200, the data driving signals outputted to each column of pixel units are inverted periodically, thereby consuming less power when compared to the LCD device 100.
Reference is made to FIG. 4 for a timing diagram illustrating the operation of the LCD device 200. In FIG. 4, SG represents the waveform of the gate driving signal, SD represents the waveform of the data driving signal, and VPIXEL represents the voltage level of the pixel unit. The grayscale value of a display image of the pixel unit is determined by the voltage difference between the data driving signal SD and the common voltage VCOM.
The gate driving signal SG is at high level during a charging period TC and a precharging period TP. The high-level gate driving signal turns on the thin film transistor switches TFT in the corresponding pixel units. The data signal SD can thus be written into the liquid crystal capacitor CLC and the storage capacitor CST in the corresponding pixel units, thereby changing the voltage levels of the corresponding pixel units.
In the prior art LCD device 200, the precharging period TP can increase the turn-on time of the thin film transistors TFT, thereby providing more time for the pixel units to reach target levels VGH or VGL. However, precharging may result in over-charging which influences the display quality. For example, if the LCD device 200 adopts NW (normally white) liquid crystal material, bright images (white images) are presented when a smaller voltage VW or no voltage is applied, and dark images (black images) are presented when a larger voltage VB is applied. Under this circumstance, over-charging occurs when a black image of a red pixel unit drives a white image of a green pixel unit, or when a black image of a green pixel unit drives a white image of a blue pixel unit. Since VB>VW, when a pixel unit displaying a black image drives a pixel unit displaying a white image, the liquid crystal material needs to be discharged, and the voltage differences established on the green and blue pixel units may not reach the ideal value for displaying the white image. Therefore, the green and blue pixel units present darker display images, which in turn cause the entire display image to be over-reddish. Similarly, if the LCD device 200 adopts NB (normally black) liquid crystal material, bright images (white images) are presented when a larger voltage VW is applied, and dark images (black images) are presented when a smaller voltage VB is applied. Under this circumstance, over-charging occurs when a white image of a red pixel unit drives a black image of a green pixel unit, or when a white image of a green pixel unit drivers a black image of a blue pixel unit. Since VW>VB, when a pixel unit displaying a black image drives a pixel unit displaying a white image, the liquid crystal material needs to be discharged, and the voltage differences established on the green and blue pixel units may not reach the ideal value for displaying the white image. Therefore, the green and blue pixel units present darker display images, which in turn cause the entire display image to be over-reddish.